Feed and treat: What to expect from commercial diets

Basic data describing the physical characteristics of fish fecal waste are important in the design of effective solid waste management in aquaculture, especially in land-based facilities such as recirculating aquacultural systems (RAS).This study describes the physical properties of feces from rainbow trout fed eight different commercially available and widely used diets in Germany. Additional data from an earlier but unpublished study pertaining to feces derived from two rather extreme all-vegetarian diets are also presented for consideration of the settling properties. The diets were tested on duplicate groups of 50 rainbow trout in a flow-through aquaculture system. The effects of the diets on the physical properties of fecal particles such as particle size distribution (PSD), modeled settling velocity and rheological character were examined and the effects of each diet on fish health, growth and feed utilization were determined. Specific growth rate (SGR) and feed conversion ratio (FCR) for the different diets ranged from 0.98% d⁻¹ ± 0.012% d⁻¹ to 1.39% d⁻¹ ± 0.012% d⁻¹ and 0.97 ± 0.017 to 1.61 ± 0.017 (mean ± S.E.), respectively. The density of presoaked feces was significantly lower than that of intestinal feces and ranged from 1.01013 ± 0.00692 g cm⁻³ to 1.04547 ± 0.00692 g cm⁻³ (mean ± S.E.). Stability data were in the range from 390.12 ± 29.4 Pa to 1214.79 ± 29.0 Pa for elastic modulus and from 62.12 ± 6.1 Pa s to 232.68 ± 6.0 Pa s for dynamic viscosity. Based on the stability and PSD data theoretical efficiencies for removal of fecal waste using a drum filter showed remarkable variation, ranging from 82.5 to 95.9% (60 μm gauze). Based on the same data, theoretical removal by a sedimentation basin with routinely using overflow rates of 0.057 cm s⁻¹ to 0.394 cm s⁻¹ ranged from 62.8 to 93.8%. Both fecal density and PSD have an exponential impact on settling performance. Increasing fecal density improves the removal efficiency of a sedimentation basin by about 20%, however sedimentation was seen to be a less robust and efficient removal technique than drum filtration. Sedimentation systems also experience additional problems with respect to leaching. Turbulence that was mimicked in this study reflects to an optimal fish farm, which means disintegrating effects are mainly caused by fish motion. If disintegrating units e.g. pumps are used, which are known to promote further particle breakdown the effects would be amplified.The results demonstrate the central importance of density of suspended solids in defining removal efficiencies and suggest that manipulation of fecal density might offer a new and effective means of managing and optimizing waste output from aquaculture operations. This study describes the basic properties of fecal wastes generated by commercial diets and can be used as a basis for further research.     

Performance of common carp,Cyprinus carpio L. and Nile tilapia,Oreochromis niloticus (L.) in integrated rice–fish culture in Bangladesh

Irrigated rice fields have enormous potential for expanding the aquaculture production in rice producing countries. Two field experiments were carried out at the Bangladesh Agricultural University, Mymensingh, to optimize the productivity of integrated rice–fish systems using Nile tilapia, Oreochromis niloticus (L.), and common carp, Cyprinus carpio L. Both experiments were laid out in a randomized complete block design with three replicates per treatment and regular rice monoculture as control. In the first trial, carp and tilapia were tested in single culture and in mixed culture with supplementary feeding at 2× maintenance level. The highest fish yield was obtained in the carp/tilapia mixed culture (586 ± 125 kg ha^− 1), followed by tilapia alone (540 ± 65 kg ha^− 1), and carp alone (257 ± 95 kg ha^− 1). Carp had significantly lower yield than the other two fish groups (p < 0.05) due to high mortality and inefficient feed utilization. As the carp/tilapia combination performed the best in the first experiment, it was tested with different inputs in the second trial, i.e. regular urea fertilization and two different feeding levels. The feeding levels were: continuous feeding at 2× maintenance level (feed level I) and a declining feeding schedule from 4× to 2× maintenance level (feed level II). The highest fish yield was obtained in feed level II (935 ± 29 kg ha^− 1), followed by feed level I (776 ± 22 kg ha^− 1), and the non-fed group (515 ± 85 kg ha^− 1). Yield differences between the treatments were significant at p < 0.05. Rice yields showed controversial effects between the rice–fish treatments and were dependent on the inputs provided. The highest rice production (4.2 t ha^− 1) was obtained from rice–fish plots with regular urea fertilization. Various significant effects of fish on water quality parameters were observed. Fish decreased the dissolved oxygen (DO) and pH value compared to rice only, especially when supplementary feed was provided. Moreover, fish stimulated the growth of phytoplankton and increased chlorophyll-a concentration. In conclusion, carp/tilapia mixed culture with supplementary feeding was found to be optimal for maximizing the output from rice–fish culture.     

Experimental investigation on the transport of different fish species in a jet fish pump

A jet fish pump with a throat of ø60 mm was designed to study its performance in the transport of different fish species and the physiological changes in fish thereafter. Experiments were conducted to investigate the fish transport rate and energy required to transport each ton of fish when transporting Carassius auratus, commonly known as the Chinese goldfish, Megalobrama amblycephala, or Wuchang bream, and Ctenopharyngodon idella, the grass carp. Fish were examined for external injuries as well as for several important enzymes and hormones which are indicators of tissue injury and stress. The results showed that the transport rate for all three species of fish rose dramatically with an increase in the primary stream rate. In this experiment, the transport rates of C. auratus, M. amblycephala and C. idella reached 2357 ± 37.2 kg ∙ h⁻¹, 2888 ± 41.6 kg ∙ h⁻¹, and 2060 ± 40.2 kg ∙ h⁻¹, respectively. However, both injury rate and energy required to transport each ton of fish increased no matter whether the primary stream rate was too low or too high. Considering both transport rate and injury rate, the mean primary stream rate of 80 m³ ∙ h⁻¹ was determined to be the optimal operating condition in this experiment. Fish were stressed and most likely some of their organs were damaged. However, most physiological indexes almost fully recovered after several hours.     

End-of-pipe denitrification using RAS effluent waste streams: Effect of C/N-ratio and hydraulic retention time

Environmentally sustainable aquaculture development requires increased nitrogen removal from recirculating aquaculture systems (RAS). In this study, removed solids from a large commercial outdoor recirculated trout farm (1000 MT year⁻¹) were explored as an endogenous carbon source for denitrification. This was done by (1) a controlled laboratory experiment on anaerobic hydrolysis of the organic matter (from sludge cones, drumfilter, and biofilter back-wash) and (2) an on-site denitrification factorial experiment varying the soluble COD (COD_S)/NO₃-N ratio from 4 to 12 at hydraulic retention times (HRT) from 50 to 170 min in simple 5.5 m³ denitrification reactors installed at the trout farm.The lab-experiments showed that the major part of the readily biodegradable organic matter was hydrolyzed within 14 days, and the hydrolysis rate was fastest the first 24 h. Organic matter from the sludge cones generated 0.21 ± 0.01 g volatile fatty acids (VFA) g⁻¹ total volatile solids (TVS), and the VFAs constituted 75% of COD_S. Analogously, 1 g TVS from the drum filter generated 0.15 ± 0.01 g VFA, constituting 68% of the COD_S. Comparison of the laboratory hydrolysis experiments and results from the on-farm study revealed as a rough estimate that potentially 17–24% of the generated VFA was lost due to the current sludge management.Inlet water to the denitrification reactors ranged in NO₃-N concentration from 8.3 to 11.7 g m⁻³ and COD_S from 52.9 to 113.4 g m⁻³ (10.0 ± 1.2 °C). The highest NO₃-N removal rate obtained was at the intermediate treatments; 91.5–124.8 g N m⁻³_reactor d⁻¹. The effect of the C/N ratio depended on the HRT. At low HRT, the variation in C/N ratio had no significant effect on NO₃-N removal rate, contrary to the effect at the high HRT. The stoichiometric ratio of COD_S/NO₃-N was 6.0 ± 2.4, ranging from 4.4 (at the high HRT) to 9.3 (at the low HRT). A simple model of the denitrification reactor developed in AQUASIM showed congruence between modeled and measured data with minor exceptions. Furthermore, this study pointed to the versatility of the NO₃-N removal pathways expressed by the bacterial population in response to changes in the environmental conditions; from autotrophic anammox activity presumably present at low C/N to dissimilatory nitrate reduction to ammonia (DNRA) at high C/N, besides the predominate “normal” heterotrophic dissimilatory nitrate reduction (denitrification).     

A novel approach for ammonia removal from fresh-water recirculated aquaculture systems,comprising ion exchange and electrochemical regeneration

A new physico-chemical process for ammonia removal from fresh-water recirculated aquaculture systems (RASs) is introduced. The method is based on separating NH₄⁺ from RAS water through an ion-exchange resin, which is subsequently regenerated by simultaneous chemical desorption and indirect electrochemical ammonia oxidation. Approach advantages include (1) only slight temperature dependence and no dependence on bacterial predators and chemical toxins; (2) no startup period is required and the system can be switched on and off at will; and (3) the fish are grown in much lower bacterial concentration, making the potential for both disease and off-flavor, lower. A small pilot scale RAS was operated for 51 d for proving the concept. The system was stocked by 105 tilapia fish (initial weight 35.8 g). The fish, which were maintained at high TAN (total ammonia nitrogen) concentrations (10–23 mgN L⁻¹) and fish density of up to 20 kg m⁻³, grew at a rate identical to their established growth potential. NH_3(aq) concentrations in the fish tank were maintained lower than the assumed toxicity threshold (0.1 mgN L⁻¹) by operating the pond water at low pH (6.5–6.7). The low pH resulted in efficient CO₂ air stripping, and low resultant CO_2(aq) concentrations (<7 mg L⁻¹). Due to efficient solids removal, no nitrification was observed in the fish tank and measured nitrite and nitrate concentrations were very low. The system was operated successfully, first at 10% and then at 5% daily makeup water exchange rate. The normalized operational costs, calculated based on data derived from the pilot operation, amounted to 28.7 $ cent per kg fish feed. The volume of the proposed process was calculated to be ∼13 times smaller than that of a typical RAS biofilter. The results show the process to be highly feasible from both the operational and economical standpoints.     

Use of artificial substrates in the culture of Litopenaeus vannamei (Biofloc System) at different stocking densities: Effects on microbial activity,water quality and production rates

Although the use of artificial substrates can favor shrimp culture, some studies indicate that their presence in growth tanks does not improve water quality or the performance of the animals. One objective of this study was to evaluate whether the presence of artificial substrates modifies the microbial activity and the water quality of the culture of Litopenaeus vannamei with bioflocs. The substrate effects on the shrimp performance and the relationship between these effects and the stocking density/biomass of shrimp were also evaluated. The experiment consisted of four treatments: D238: 238 shrimp m⁻³; D238 + S: 238 shrimp m⁻³ + substrates; D473: 473 shrimp m⁻³; D473 + S: 473 shrimp m⁻³ + substrates. Twelve experimental units of 850 L were stocked with juvenile L. vannamei (2.6 g) that were grown for 34 days. The substrates did not appear to affect water quality since the concentrations of orthophosphate, ammonia and nitrite were not significantly different in tanks with or without substrates. The periphyton biomass was low and the biological activity on the substrates was not significant, indicating that the water quality variables were mainly controlled by the microbial community associated with the suspended bioflocs. The shrimp grown in the presence of the substrate exhibited greater weight gain (D238 + S = 1.40 ± 0.05 and D473 + S = 1.20 ± 0.04 g week⁻¹) than those grown without substrates (D238 = 0.73 ± 0.04 and D473 = 0.44 ± 0.13 g week⁻¹). The final biomass was 314% greater in the tanks with substrates. The shrimp survival was significantly higher in the tanks with substrates (93.9 ± 2.4%) than in the tanks without substrates (42.5 ± 35.9%). The results indicate that the substrates served to increase the surface area of the tank and to reduce the relative stocking density, which appears to reduce the stress levels of shrimp, indicated by higher shrimp performance. In tanks with higher biomass, where the negative effects of intensification were most severe, the presence of the substrates had a positive effect on the production indices.     

Performance of single-drain and dual-drain tanks in terms of water velocity profile and solids flushing for in vivo digestibility studies in juvenile shrimp

In vivo digestibility determination in shrimp is a challenge because these animals are coprophagous, benthic and slow feeders and the small amount of feces that they produce is difficult to collect. The objective of this study was to evaluate an efficient tank design for the purpose of studying shrimp digestibility. Different tank designs were evaluated considering drain system (dual-drain and single-drain), water inlet flow rate (8, 12, and 16 L min⁻¹) and bottom drain diameter (6, 13, 19, 25 and 50 mm) and their effects on tank hydraulics, water velocity and solids flushing. A circular and slightly conical 500 L tank was adapted with a clarifier for the two dual-drain designs (Cornell-type and central-type) and settling columns for the two single-drain designs (Guelph-F and Guelph-L). Results showed that: (1) water rotational velocity profile was more homogeneous in tanks with larger bottom drain outlets, and water velocity increased with water inlet flow rate from almost zero up to 14.5 ± 0.7 cm s⁻¹; (2) solids flushing, measured as the percentage of feed pellets retained at both the bottom drain and in the settling devices, was positively correlated with the surface loading rate (L min⁻¹ flow per m²) and was more effective at the Guelph-L design fitted with a 150 mm diameter settling column. In this system 100% of the solids were removed at the inflow rate of 16 L min⁻¹. It can be concluded that among the systems evaluated, the Guelph-L at an inflow-rate of 12 L min⁻¹ was most efficient for both solids removal and water velocity profile and thus seemed more suitable for shrimp digestibility studies in high performance conditions. Technologies involving hydrodynamic must be intensively applied to solids removal for aquatic species production as well as research purposes like digestibility, which is highlighted in this study.     

Water delivery capacity of a vacuum airlift – Application to water recycling in aquaculture systems

A study was undertaken to measure the water flow (Q_w) delivered by a vacuum airlift designed for recirculating aquaculture systems (RAS) in fresh (<1‰ of salinity) and sea water (35‰ of salinity). The vacuum airlift consists of two concentric tubes connected at their top to a depression chamber. The water rises in the inner tube as a result of air being injected in its lower section and flows back through the external downcomer tube. The vacuum airlift was adjusted at three different lengths: 2, 4 or 6 m and water discharge could be lifted from 0 to 30 cm. Air flow rate (Q_g) varied from 0 to 80 L min⁻¹. Different types of air injectors were tested, delivering different bubble sizes (0.1–5 mm) depending on porosity and functioning at low or high injection pressure. Results show an increase in water flow when pipe length and air flow were increased and lift height reduced. Water flow also depended on the type of water and ranged from 0 to 35 m³ h⁻¹ (0–580 L min⁻¹) for fresh water and only from 0 to 20 m³ h⁻¹ (0–330 L min⁻¹) for sea water (for a 6 m high vacuum airlift). This difference was attributed to the smaller bubble diameter and higher gas holdup (ɛ_g) observed in sea water (0–20%) compared to fresh water (0–10%). When bubbles were present in the downcomer tube, they created a resistance to flow (counter-current airlift) that slowed down liquid velocity and thus water flow. Increasing the vacuum made it possible to use low air injection pressures and high injection depths. Vacuum also increased bubble size and airflow (20 L min⁻¹ at atmospheric pressure to 60 L min⁻¹ at 0.3 barA) and thus water flow rates. With RAS, the presence of fish feed in water rapidly increased water flow delivered by the airlift because of changes of water quality and gas holdup. When working with low head RAS (under 0.3 m), vacuum airlift could save up to 50% of the energy required for centrifugal pumps. An empirical predictive model was developed and calibrated. Simulation shows a good correlation between predicted values and measurements (R² = 0.96).     

Bio-flocs technology application in over-wintering of tilapia

A 50-day experiment was conducted to investigate the effectiveness of the bio-flocs technology for maintaining good water quality in over-wintering ponds for tilapia hybrid fingerlings (Oreochromis niloticus × Oreochromis aureus). To preserve adequate water temperatures in the ponds, they were covered with polyethylene sheets and the water exchange rate was minimized to increase pond water temperature. To avoid water quality deterioration, starch was added to the ponds to stimulate the formation of bio-flocs. Temperature in the covered ponds could easily be controlled and was 0.4–4.9 °C higher than the influent water. Adjusting the C/N ratio in the ponds by adding starch or increasing the amount of carbohydrates added through the feed limited the presence of inorganic nitrogen species when the C/N was about 20, even at high stocking densities of 20 kg/m³ at harvest. Fish survival levels were excellent, being 97 ± 6% for 100 g fish and 80 ± 4% for 50 g fish. Moreover, at harvest the condition of the fish was good in all ponds with a fish condition factor of 2.1–2.3. Overall, these findings can help to overcome over-wintering problems, particularly mass mortality of fish due to low temperatures in the ponds.     

Leaching properties of three different micropaticulate diets and preference of the diets in cod (Gadus morhua L.) larvae

Leakage of water soluble nutrients from larval microparticulated feeds is probably extensive and needs to be further investigated. Leaching rates of ¹⁴C-labelled serine, pepsin hydrolysed, protein enriched ¹⁴C-algae extract and intact protein enriched ¹⁴C-algae extract were measured from three microparticulated feeds for marine fish larvae (heat coagulated, protein bond feed; agglomerated feed; protein encapsulated feed). The effects of particle size (< 0.3 mm, 0.3–0.6 mm; 0.6–1.0 mm) and immersion time (1–60 min) in salt water were also tested. Leaching increased by decreasing molecular weight of leaching component (P < 10^− 5), by the feeds in order encapsulated, heat coagulated and agglomerated feeds (P < 10^− 5), by longer immersion time (P < 10^− 5), and by decreasing feed particle size (P < 10^− 5). Due to low protein content of the algae extract, the leaching rates of intact and hydrolysed algae extract did not represent absolute estimates for protein and hydrolysed protein leaching. A new estimate for leakage of hydrolysed protein was calculated based on molecular weight distribution of the hydrolysed algae extract analysed by cutoff centrifugation of the extract. Assuming that molecules < 300–600 or < 9–18 kD would leak, leakage of hydrolysed protein from the smallest feed particles after 5 min immersion would be 80–98%, 43–54% and 4–6% of the agglomerated, heat coagulated and protein encapsulated feeds, respectively. The feeds were also tested for preference in cod larvae of two different sizes (5.6 ± 2.5 mg and 15.8 ± 7.2 mg). The preference was highest for the heat coagulated feed in the first experiment (feed intake 0.32 ± 0.06 mg dry feed larvae^− 1) and the agglomerated in the second (2.04 ± 0.32 mg dry feed larvae^− 1), while the protein encapsulated feed was preferred at lower rates in both experiments (0.11–0.14 mg dry feed larvae^− 1).     

Nutrient discharge,sludge quantity and characteristics in biofloc shrimp culture using two methods of carbohydrate fertilization

The aim of this study was to evaluate the effect of two methodologies of carbohydrate fertilization on the volume and characteristics of effluent from intensive biofloc shrimp cultivation. Six fiberglass circular tanks (50 m² each) were divided into two treatments. In the treatment called continuous (CONT), the tanks received daily molasses fertilization throughout the entire rearing period. In the treatment named initial (INI), molasses was used only in the early weeks of cultivation. Juvenile Litopenaeus vannamei (0.87 ± 0.10 g) were stocked at a density of 180 animals m⁻² and cultured during 12 weeks until they reached an average weight of 12 g. The tanks were operated with no water exchange and the total suspended solids concentration were kept between 300 and 400 mg L⁻¹ using settling chambers. The sludge produced and the wastewater at harvest were quantified and their characteristics were determined. The production of TSS in the CONT treatment was higher (0.25 kg of solids per kg of applied feed) than in the INI treatment (0.16 kg kg⁻¹) (P < 0.05). The analysis of the sludge revealed a high amount of volatile solids in both treatments, between 636 and 702 g kg⁻¹. However, due to the elevated sludge nitrogen content, the carbon to nitrogen (C:N) ratio was low, with values of 6.4 ± 1.4 and 7.5 ± 1.6 for INI and CONT respectively. The BOD:TSS ratio was also low in both treatments, but the INI showed lower values (10.3 ± 0.6%) than the CONT (14.9 ± 0.0%) (P < 0.05). Both fertilization strategies were able to modify the characteristics of sludge produced during cultivation. Moreover, the high nitrogen and sulfate content of the sludge in both treatments indicated that it may be difficult to use an anaerobic digestion process to treat sludge. In the INI treatment tanks, the sludge is partially stabilized, while in the CONT there was a greater need for stabilization.     

Comparative growth and welfare in rainbow trout reared in recirculating and flow through rearing systems

The objective of this study was to compare fish performance and welfare at different stocking densities in a recirculating system (RS) and a flow through system (FTS) under field conditions. During the 77 days experiment, the fish survival rate was high (99.3%) and stocking density increased from 57 to 98–108 kg m^?3. No significant differences in growth were observed between RS and FTS until day 56. Later, growth decreased in the FTS, while it remained similar to the farm reference at 50 kg m^?3 in the RS. Final weight was 17% higher in RS than in FTS. The maximum carrying capacity of the RS was near 100 kg m^?3, limited by NO₂ increase up to safe level at the end of the experiment, the maximum carrying capacity of the FTS was near 85 kg m^?3, probably limited by CO₂ concentration (17.8 ± 5.7 mg l^?1). In the RS, the relative length index of pectoral and dorsal fins was lower than in the FTS, which may be attributed to the tank hydrodynamics. In both systems, an improvement of the pectoral and dorsal profile was observed at the end of the experiment, attributed to a swimming activity reduction that may have decreased contact between individuals. In the RS, high caudal fin deterioration (50% versus 20% in FTS) was observed irrespective of stocking density, that could be linked to the highest water velocity modifying the fish swimming activity. The results confirm that when water quality is maintained in safe level ranges, high densities can be used in trout RS without fish performance and pectoral or dorsal fin deterioration, but with a major caudal impairment.     

The efficiency of three fry trawls for sampling the freshwater pelagic fry community

Avoidance reactions of young-of-the-year fish assemblages were investigated in the offshore zone of three reservoirs in the Czech Republic during late summer. Pikeperch (Stizostedion lucioperca) strongly dominated in the Lipno Reservoir in 2003, bream (Abramis brama) and bleak (Alburnus alburnus) were the most abundant species in the Slapy Reservoir in 2004 and roach (Rutilus rutilus) and bleak dominated in the Želivka Reservoir in 2004 and 2005. Densities of fry ranged between 1.09 ind./100 m³ in the Lipno Reservoir and 2.95 ind./100 m³ in the Slapy Reservoir. In each reservoir, three trawl nets of different widths (dimensions 1.5 m × 3 m, 3 m × 3 m, 6 m × 3 m) were used in the pelagic area at night. For each trawl the estimated fish density was determined as the catch per 100 m³ of water and was compared between all three trawls. The fish length–frequency distribution was also compared between all the trawls. Day catches were much lower compared with those taken at night. Adult fish showed very clear avoidance of fry trawls. At night, no important differences in estimated fry density, obtained by these trawls, were observed in any locality for any fish species, nor in the length distribution of important species between trawls. According to these results, the avoidance reactions of fry in the late summer (August, September), at night, are unimportant. At this time of year, nighttime pelagic trawling appears to be an effective method for quantitative fry sampling. We recommend the 3 m × 3 m trawl for quantitative night sampling of the fry community as a good compromise between assured efficiency and relatively easy handling.     

Does orientation of raft helps in augmenting yield during lean period?: A case study of Gracilaria edulis cultivation in open sea by vertical raft alignment along the south-eastern coast of India

The food grade agar in India has been almost exclusively obtained from Gracilaria edulis, but the industrial production overwhelmingly relies on exploitation of natural resources. United Nations efforts through Food and Agriculture Organization under Bay of Bengal Program highlighted the necessity of undertaking commercial farming of this species along Indian coast for socio-economic benefits. The pilot-scale experiments established viability of large-scale cultivation by floating raft method. Nevertheless, drastic reduction in yield and quality during summer months due to enhanced sedimentation and severe epiphytism is found to be a major hindrance. Altering the positioning of rafts from horizontal to vertical alignment improved the growth and yield under open sea condition at two different locations along south east coast of India. The average yield in horizontal raft was found to be 3.08 ± 0.61 kg fr wt raft⁻¹ with corresponding DGR of 1.87 ± 0.63% day⁻¹ while same in case of vertical rafts was 13.76 ± 3.86 kg fr wt raft⁻¹ and 5.00 ± 0.5% day⁻¹ in Gulf of Mannar under 45 days growth cycle. The corresponding values along Palk Bay were 2.98 ± 0.52 g fr wt raft⁻¹ and 1.38 ± 0.42% day⁻¹ for horizontal raft and 13.02 ± 6.06 kg fr wt raft⁻¹ and 4.14 ± 1.18% day⁻¹ for vertical raft. ANOVA clearly indicated that raft position significantly influenced the biomass yield and DGR at Palk Bay (F = 75.77; F = 112.81) as well as Gulf of Mannar (F = 27.21; F = 59.16) at p = 0.001. The increment of 1.9–2.6% in fresh weight of individual frond was reported in vertically aligned rafts. The computational fluid dynamics (CFD) based unsteady numerical simulations have confirmed that vertical alignment of raft facilitates relatively free movement of water due to which sedimentation and epiphytism are either minimised or eliminated. Thus these studies can help us to deduce important conclusions pertaining to management of sustained commercial cultivation of this alga in Indian waters.     

The effect of density on sea bass (Dicentrarchus labrax) performance in a tank-based recirculating system

Sea bass (Dicentrarchus labrax) (135 ± 4 g) were reared under tank-based recirculating aquaculture system for a 63-day period at four densities: 10, 40, 70, 100 kg m^?3. Fish performance, stress indicators (plasma cortisol, proteonemia plus other blood parameters—Na⁺, K⁺, glucose, pH, total CO₂^?) and water quality were monitored. At the end of the 63-day period, resistance to infection was also studied by a nodavirus challenge. A 25-day test was performed on fish from two extreme densities (10 and 100 kg m³) and one intermediate density (40 kg m³).With regards to the different density treatments, there was no significant difference between the daily feed intake (DFI) and the specific growth rate (SGR) up to a density of 70 kg m^?3. No significant difference was found between treatments concerning the feed conversion ratio (FCR) and the mortality rate. No density effect was observed on the fish stress level (plasma cortisol) or on sensitivity to the nodavirus challenge. Under these experimental rearing conditions, the density above 70 kg m^?3 has an impact on growth performance (DFI and SGR) indicators and also some blood parameters (CO₂) at the highest density tested (100 kg m^?3).This study suggests that a density up to 70 kg m^?3 has no influence on sea bass performance and welfare. At 100 kg m^?3, average specific growth rate was decreased by 14% without welfare deterioration according to the welfare indicators monitored.     

The “self cleaning inherent gas denitrification-reactor” for nitrate elimination in RAS for pike perch (Sander lucioperca) production

Denitrification reactors have proven their functionality in commercial recirculation aquaculture systems (RAS). Nevertheless, clogging occurs due to the low hydraulic loads necessary to accomplish anoxic conditions for a successful denitrification process in RAS, which hampers the adjustment of stable working conditions within fixed bed denitrification reactors. Reactors working on the basis of activated sludge demand careful hydraulic control and/or complex configurations for sludge retention.To develop a low-maintenance denitrification reactor, an enclosed moving bed filter, driven by recirculation of the inherent, oxygen poor gas was designed. A Self cleaning Inherent gas Denitrification reactor (SID-reactor) of 0.65 m³, which offered a moving bed volume of 0.39 m³ was connected with a RAS of semi-industrial scale for pike perch (Sander lucioperca) production. This species indicates suboptimal environmental conditions (as e.g. NO₃-N concentrations above approximately 68 mg l⁻¹) by prompt reduction of the feed intake. In different experimental series, the SID-reactor was operated with denatured ethanol, methanol, acetic acid or glycerin as carbon sources and changing operational modes.Clogging was prevented by a 40 second inherent gas recirculation twice an hour, which provided continuous, maintenance free operation with marginal energy demand. With inlet (RAS) and outlet NO₃-N concentrations in the range of 49 mg l⁻¹ and 12 mg l⁻¹, mean denitrification rates of 199 g to 235 g NO₃-N per m³ moving bed volume and day were determined for all tested carbon sources. Negative effects on the feed intake of the reared pike perch were detected with all carbon sources except methanol. Changing the mode of operation to continuous circulation of the filter bed at inlet NO₃-N concentrations of 26 mg l⁻¹, the denitrification performance reached 451 g NO₃-N per m³ moving bed volume and day. The SID-reactor allowed for the reduction of freshwater exchange in the pike perch RAS from 600 l to 70 l (−88%) and the sodium bicarbonate buffer from 182 g to 31 g (−83%) per kg of administered food. The easy and reliable operation of the SID-reactor could help to establish controlled denitrification as a routine purification step in RAS.     

Performance evaluation of four different methods for circulating water in commercial-scale,split-pond aquaculture systems

Split-pond aquaculture systems are being implemented by United States (US) catfish farmers as a way to improve production performance. The split-pond consists of a fish-culture basin that is connected to a waste-treatment lagoon by two water conveyance structures. Water is circulated between the two basins with high-volume pumps (water circulators) and many different units are being used on commercial farms. In this study circulator performance was evaluated with four different circulating systems. Rotational speeds ranged from 0.5 to 3.5 rpm for a twin, slow rotating paddlewheel; 12.5 to 56.5 rpm for a paddlewheel aerator; 60 to 240 rpm for a high-speed screw pump; and 150 to 600 rpm for an axial-flow pump. Water flow rates ranged from 8.6 to 77.6 m³/min and increased with increasing rotational speed. Power input varied directly with flow rate and ranged from 0.24 to 13.43 kW for all four circulators. Water discharge per unit power input (i.e., efficiency) ranged from 3.5 to 70.9 m³ min⁻¹ kW⁻¹ for the circulators tested. In general, efficiency decreased as water flow rate increased. Initial investment cost for each circulator and complete circulating system ranged from US $5850 to $22,900, and $15,335 to $78,660, respectively. The least expensive circulator to operate was the twin, slow-rotating paddlewheel, followed by the paddlewheel aerator, high-speed screw pump, and axial-flow pump. Our results show that four different circulating systems can be effectively installed and used to circulate water in split-ponds. However, water flow rate, rotational speed, required power input, efficiency, initial investment cost, and operational expense varied greatly among the systems tested. Long term studies are underway to better define the relationship between water flow rate and fish production in split-ponds. That information will help identify the water circulating system most appropriate for split-pond aquaculture.     

Routine oxygen consumption rates of california halibut (Paralichthys californicus) juveniles under farm-like conditions

Fish oxygen requirement is a fundamental variable of aquaculture system design and management, as it is the basis for determining water flow rates for sustaining stock. A study on oxygen consumption of California halibut (Paralichthys californicus) between 3.2 and 165.6 g was conducted in small raceways (2.41 m long, 0.28 m wide, and 0.22 m high; operational water depth between 0.05 and 0.10 m with a quiescent zone 19 cm long in the effluent section) working as open respirometers in a recirculating system under farm-like conditions. The fish were fed commercial dry pelleted feeds at a ratio of ∼0.70–3.00% of body weight (BW) and stocked at densities between 94% and 316% percent coverage area (PCA). Oxygen consumption rates were determined by mass balance calculations. The mean and maximum oxygen consumption rates (g O₂/kg fish/day) for juvenile California halibut under the conditions tested can be expressed by M_day = 15.077W^−0.2452 and M_day = 17.266W^−0.2033, respectively, where W is fish weight in grams. The determination of oxygen consumption by California halibut in farm-like conditions provides valuable information on the oxygen requirement of these fish in an aquacultural setting. This information can be used for designing and sizing a rearing facility for the intensive culture of California halibut.     

Pulse versus continuous peracetic acid applications: Effects on rainbow trout performance,biofilm formation and water quality

Peracetic acid (PAA) products are being introduced to aquaculture as sustainable disinfectants. Two strategies are used to apply PAA: high dose pulse applications, or low dose continuous application. In the present study, their impacts on fish health and water quality were investigated in triplicate flow-through tanks stocked with rainbow trout. The gentler and shorter water cortisol increase measured along twice-per-week pulse applications of 1 mg L⁻¹ PAA indicated a progressive adaptation of fish. In contrast, the continuous application of 0.2 mg L⁻¹ PAA caused no stress to fish. Meanwhile, no mortality and no impact on growth or innate cellular immunity were observed. The pulse applications restricted biofilm formation, and partially inhibited nitrification. Additionally, the highest oxygen concentration and stable pH were observed. In contrast, the continuous application promoted biofilm formation, and caused a pH increase and intermediate oxygen concentration. The contrast was probably due to different susceptibility of microbes to PAA-induced oxidative stress. To summarize, pulse PAA applications cause minor stress in fish, but have advantages over continuous application by ensuring better water quality.     

Direct and continuous dissolved CO2 monitoring in shallow raceway systems: From laboratory to commercial-scale applications

Direct and continuous measurement of dissolved CO₂ (dCO₂) is crucial for intensive aquaculture, especially in shallow raceway systems (SRS). In this work the performance of a portable dissolved CO₂ probe analyzer was tested for the effects of different aqueous solutions, pure oxygen injection and agitation. Laboratory results showed significant (p < 0.05) solution effects on probe performance for low (10–20 mg L⁻¹) and high (30–50 mg L⁻¹) dCO₂ concentrations. Globally performance was better in deionized water, followed by marine fish farm water and artificial seawater. Accuracy and response time were the parameters most affected by the type of solution tested. Linearity was always observed (R² = 0.995–0.999). The probe was sensitive to 1 mg L⁻¹ dCO₂ increments for concentrations <6 mg L⁻¹ in artificial seawater. Pure oxygen injection did not affect probe readouts, and agitation was needed for better accuracy and response time. In real marine SRS with tanks in series dCO₂ dynamics was revealed using the probe coupled to a developed flow cell. A prototype SRS was built and used to study dCO₂ dynamics without endangering cultivated fish. Generally, results obtained indicate that the probe tested although precise, is better suited for discrete, single-point dCO₂ monitoring, being a limited resource for the special needs of shallow raceway systems. As SRS represent a paradigm change in aquaculture, new water quality monitoring strategies and instrumentation are needed, especially for dCO₂. Fiber optic sensors can be a solution for continuous, multipoint monitoring, thus contributing to the understanding of water quality dynamics in hyperintensive aquaculture systems.